The listings are supposed to be removed by the person/agent who sets up the listing feed to Zillow. As you can see, the feed source is still active so it will come off Zillow when the listing source becomes inactive. One can flag a property that is no longer for sale (go to "report problem with listing" at the bottom of the home details page). Zillow doesn't monitor whether or not agents manage their listings properly (that would be tough with millions of listings and 300 employees), but does make the agent renew them periodically and removes them when reported.